CHAPTER XATMOSPHERIC OPTICS

Photographic Analysis of Lightning.This double photograph was made by Dr. B. Walter of Hamburg, the leading expert in the field of lightning photography. The picture at the left was taken with a stationary camera. The photograph at the right, taken at the same time with a revolving camera, shows that one of the main flashes (the one to the right) was a single discharge, and the other a multiple discharge.

From any distant point on the earth’s surface either north or south of it, the visible portion of the auroral ring presents the appearance of an arch across the horizon. Arctic explorers, far within the Arctic Circle, see this arch to the south of them. In our latitudes it spans the northern horizon. Separate beams or streamers may be distinguishable ornot, according to the brightness of the discharge or its distance from the point of observation. Combinations of beams constitute so-called “draperies.”

Photograph of Lightning, Showing “Black” Flashes.(graph by F. Ellerman.)

Atmospheric Electricity Instruments on Board the “Carnegie.” Left: Penetrating radiation apparatus. Right: active-content apparatus. Below: Arrangement for supplying potentials to electroscopes and ionization chambers, (Carnegie Institution.)

Occasionally, at times of great solar activity, part of the ring actually overlies our Northern States, and the aurora then becomes a magnificent spectacle in this part of the world. The whole sky may be filled with the shifting streamers, along which travel rapid pulsations of light, so that the phenomenon then suggests strongly what it really is—a vast electrical discharge passing down through the atmosphere from outer space. When the observer is thus surrounded by the beams, they seem, on account of perspective, to converge toward a point south of the zenith, where they form a beautifulcoronaor crown. The position of this crown depends upon the slant of the beams, which, as already explained, follow the lines of force.

A brilliant aurora is always accompanied by disturbances of the magnetic needle, which moves about erratically, so that compasses can no longer be depended upon. At the same time there are strong “earth currents,” which interfere with the operation of telegraph lines.

Observations with the spectroscope seem to show that the light of the aurora is chiefly due to glowing nitrogen, though the most prominent line in the auroral spectrum has sometimes been referred to an unknown atmospheric gas. The various colors seen in bright auroras, including reds, greens, and yellows, are believed by some authorities to depend upon the varying speed of the electrical discharge. Experiments with vacuum tubes show that nitrogen, especially, gives great changes of color with changes in the velocity of the discharge. Another interesting revelation of the spectroscope is that there isapparently, a faint auroral illumination in the sky at all times and in all parts of the world, the so-called “permanent aurora.”

Photography has been used with great success in studying the aurora, especially by the Norwegian physicists Störmer, Vegard, and Krogness. Simultaneous photographs of a single detail are taken from two points several miles apart against a background of stars. The apparent position of the auroral detail among the stars will differ in the two pictures, and a comparison of them makes it possible to determine the actual position of the aurora in space. A slow-moving cinematograph has also been used to obtain series of pictures. The measurements of these observers show that the base of the aurora is, generally between 60 and 70 miles above the earth with a strong tendency to assume a definite location at an altitude of about 61 or 67 miles. Its upper limits are not well defined, but it has been photographed up to an altitude of more than 300 miles. Earlier observers reported seeing the aurora at altitudes of only a few miles, and even down to the earth’s surface, but recent authorities are inclined to discredit these observations.

One more phenomenon of atmospheric electricity requires brief mention, viz., the electric waves that produce the erratic disturbances known to wireless telegraph operators as “strays” or “static.” As heard in the receiver of a wireless outfit the noise of strays has been described as “like hailstones beating against a sheet of tin,” or “short hisses from a steam pipe,” or “periodic discharges of coal down a chute.” Another characteristic sound is a sharp “click.” The study of strays has been carried out on a world-wide scale by a committee of the BritishAssociation for the Advancement of Science, but their nature is not yet fully understood. Some strays are undoubtedly due to near or distant discharges of lightning, and special forms of wireless apparatus, known as “thunderstorm recorders” or “ceraunographs,” have been used to give notice of the approach of thunderstorms. On the other hand, strays seem frequently to have no connection with thunderstorms, and their principal origin is now sought in electrical disturbances in the upper atmosphere, perhaps similar to those which cause the aurora, and, as in the case of the aurora, having their ultimate source in the sun.

Whenwe look up into the sky on a cloudless day we behold a continuous canopy, the prevailing color of which is blue. This canopy is a veil that hides the starry hosts beyond, and its presence seems, at first sight, incompatible with the fact that the air is a transparent medium. We see the stars by night through the same intervening atmosphere. Why are they cut off from our sight by day? The answer to this question can, perhaps, best be made plain by a simple experiment. Place a lighted candle behind a sheet hung across a room not otherwise illuminated. The flame of the candle will be distinctly visible through the sheet. Next, let the room be brightly lighted, say with electric light or daylight. The candle can now no longer be seen through the sheet, owing to the bright illumination of the latter as compared with the feeble light of the candle.

In the atmosphere the counterpart of our sheet is a layer, several miles in depth, of minute particles, which by day are lighted up by the sun. Some of the particles are tiny dust motes, others are fine droplets of water or bits of ice, and the rest are the molecules of the atmospheric gases themselves. It is the light that comes to us from these particles that makes our eyes insensitive to the fainter light of the stars, and makes the sky itself a visible luminous vault.

Next, why is the clear sky generally blue, rather than some other color? To answer this question, we must recall the fact that sunlight is made up of ether waves of many different sizes. In combination, these waves produce upon our eyes the sensation of white light. When they are separated, as by passing through a prism, the smallest waves—or, in more technical language, the vibrations of shortest wave length—register the sensation of violet, and the largest or longest waves that of red. The whole sequence of colors runs in the order violet, indigo, blue, green, yellow, orange and red (easily fixed in the memory by means of the word VIBGYOR, formed from the initial letters of these words).

Now the passage of sunlight through the atmosphere is obstructed to a certain extent, not only by suspended dust particles, but also by the molecules of the air. Let us consider, first, the effect of air molecules and of the finest dust particles, not much above molecular size. These tiny objects have different effects on light waves of different lengths. The longest waves are little disturbed by them, just as ordinary waves in water are little affected by a floating cork, for instance. The shortest waves are so small in proportion to the size of the obstacles that they are diffused or scattered by them, as a tiny ripple in water might be broken up by a floating cork. It is this diffuse light, of short wave length, that gives the sky its color. A large part of the violet and indigo light is lost by further scattering before it reaches the earth, leaving a preponderance of blue in the sky as we see it. When the air contains a considerable amount of suspended particles larger than those above considered—whether in the form of solid dust or crystals of ice or tiny droplets ofwater—light of all wave lengths is reflected by them, and the sky looks white or grayish.

On account of the action of atmospheric particles in filtering out the shorter light waves, as just described, sunlight becomes relatively rich in red and orange in passing through the air. When the sun is high, the path of the sunbeams to the earth is short, and the color of their light is but little affected. Near the time of sunrise and sunset, however, sunlight comes to us through a much greater extent of air, and the filtering process is much more effective. Hence the sunshine is both enfeebled and reddened when the sun is near the horizon. The diffuse light of the sky around the sun is filtered in the same manner, and therefore is commonly red when the sun is low.

A gray sunset sky after a clear day is due to the presence of water drops in the air, and indicates conditions favorable for rain, since, unless the air were saturated to a considerable altitude, the comparatively warm sunshine of the afternoon would favor evaporation rather than condensation of moisture. A gray sunrise sky has, as a general rule, just the opposite meaning. It often indicates the presence in the air of water drops formed on dust particles during the night, after the manner of dew, because the upper air has beendryenough to permit rapid radiation from the dust. These drops will be speedily evaporated by the rising sun, and the general dryness of the atmosphere will not favor further condensation. Several familiar weather proverbs are thus justified, e. g.:

Evening red and morning grayHelp the traveler on his way;Evening gray and morning redBring down rain upon his head.

There are many other interesting optical phenomena connected with sunrise and sunset, including, first of all, the morning and evening twilight. When the sun, or any other heavenly body, is only a little below a clear horizon, it is still visible, on account of the bending of its rays by the atmosphere. This lifting effect, known asastronomical refraction, amounts to about half a degree (at the horizon), which is about equivalent to the apparent diameter of the sun or moon. As the sun sinks farther below the horizon, in the evening, the only daylight that comes to us is that reflected from the upper levels of the atmosphere, which are still illuminated. This is called twilight, and it lasts until the sun is about 18 degrees below the horizon, when total darkness sets in. The period as a whole is sometimes calledastronomical twilight, in distinction from the briefer period known ascivil twilight, during which there is light enough for outdoor occupations; the latter lasts from sunset until the sun is about 6 degrees below the horizon. Morning twilight is more commonly called “dawn.”

An interesting succession of light and color effects is observed before and after sunset and, in inverse position and order, about sunrise. Considering sunset only: After the sun has sunk out of sight, a broad band of golden light, called thebright segment, is seen along the western horizon. Above this, in the western sky, appears a more or less circular expanse of rosy glow, known as thepurple light. In the eastern heavens, after sunset, there rises steadily from the horizon the so-calleddark segment, which is the blue or ashy shadow of the earth on the sky. This is bordered above by the pink or purplishantitwilight arch. As time goeson, the purple light in the west, after increasing in brightness for a while, finally sinks behind the bright segment; while in the east the rising dark segment encroaches upon and finally obliterates the antitwilight arch. Sometimes, in clear weather, there is a fainter repetition of these lights and colors (second purple light, etc.).

Among the Alps and other snow-capped mountains, these sunset and sunrise phenomena assume a particularly beautiful form, known as theAlpenglow. In fine weather, just before sunset, the peaks to the eastward begin to show a reddish or golden hue. This fades gradually, but in a few minutes, when the sun is a little below the horizon of the observer, but the peaks themselves are still bathed in direct sunlight, an intense red glow, beginning down the slopes, moves upward to the summits. This is identical with the antitwilight arch described above. Presently this glow is succeeded by an ashy tint, as the peaks are immersed in the rising shadow of the earth (the dark segment). Their rocks and snows assume a livid appearance, aptly described by the inhabitants of Chamonix, whence the phenomena in question are well seen on the summit of Mont Blanc, as theteinte cadavéreuse. In ordinary weather darkness succeeds without any further notable phenomena, but occasionally there occurs a remarkable renewal of rosy light upon the peaks, known as therecolorationorafterglow. At Chamonix this is termed the “resurrection of Mont Blanc.” The afterglow has been variously explained, but it is probably due, mainly at least, to the reflection of the purple light in the western sky. Sometimes it lasts until an hour after sunset, and it passes away from below upward. On very rare occasions there is a secondafterglow, presumably the reflection of the second purple light mentioned above. Similar phenomena are often seen in reverse order at sunrise.

A pretty phenomenon observed chiefly in the late afternoon and early morning consists of beams of light radiating from the sun, known technically ascrepuscular rays. The beams are made visible by the presence of abundant dust or water droplets in the atmosphere, and the intervening dark spaces are the shadows of clouds. When the sun is above the horizon and the beams are directed downward, the phenomenon is popularly described as “the sun drawing water” and is regarded as a sign of rain. Sailors call these beams the “backstays of the sun,” and they have several other names based upon the legendry associated with them in different parts of the world. After sunset or before sunrise a fanlike sheaf of the beams often extends upward from the western or eastern horizon, respectively. The Homeric expression “rosy-fingered dawn” probably refers to this phenomenon. In all cases the apparent divergence of these beams is an effect of perspective, as they are really parallel. A rarer phenomenon is that ofanticrepuscular rays, which appear to converge to a point opposite the sun. In this case the beams and shadows are projected entirely across the sky, but their paths can very seldom be traced in the upper part of the heavens because in this direction the observer’s line of sight passes through a comparatively shallow extent of dusty atmosphere.

An analogous phenomenon is seen in the shadows which near-by isolated mountain peaks frequently cast upon the sky opposite the sun at sunrise and sunset. Travelers have described such shadows castby Adam’s Peak in Ceylon, Pike’s Peak in the Rocky Mountains, and Fujiyama in Japan. The phenomenon is said to be especially striking in the polar regions, where the air is often heavily charged with particles of ice.

One more optical phenomenon of sunrise and sunset that requires mention here seems to be comparatively little known to the nonscientific public, notwithstanding the fact that it has supplied the subject and title of a diverting novel by Jules Verne. The conditions required for its appearance are a clear and steady atmosphere and a sharply defined horizon, such as that of the ocean. At the instant the sun is appearing or disappearing, and when only a very small segment of its disk is visible above the horizon, this portion appears to be colored a bright emerald green, sometimes blending into blue. This transient phenomenon is known as thegreen flash. It is best explained as due to the different degrees of refraction undergone by rays of different wave lengths coming to us from the sun. The effect of refraction in elevating the solar image as a whole when near the horizon has already been mentioned. This effect is a little greater for the green and blue rays than for the orange and red. It is still more pronounced for the violet and indigo rays, but these are mostly sifted out of the solar beams in their long passage through the atmosphere when the sun is low. Hence at the upper edge of the solar image there is a narrow green or blue fringe, which is not, however, perceptible except when a screen is interposed between the eye and the bright image of the sun. A sharp horizon furnishes such a screen. Through a telescope it is possible, in suitable weather, to see the green flash—and also a corresponding“red flash” at the lower edge of the sun—by placing an opaque diaphragm in the focal plane of the object glass. Another explanation of the green flash—which could, however, account only for its appearance at sunset and not at sunrise—is that it is a physiological effect; the eye, fatigued by the reds and yellows that predominate in the light of the setting sun, sees an “after image” of complementary hue the instant after the real image has disappeared.

We now turn to a group of phenomena, also due to atmospheric refraction, which includes some of the most bizarre of optical illusions. The simplest of these phenomena consists of a slight apparent elevation of all objects in the surrounding landscape throughterrestrial refraction, which is identical in principle with astronomical refraction and depends upon the difference in density, and hence of refractive power, of the air at different levels above the earth.

Normally the air decreases in density at a nearly regular rate with increasing altitude. Sometimes, however, this change in density is greatly modified by local effects of temperature. Over a cold surface of land or water the adjacent air may be abnormally dense, resulting in an unusually rapid decrease of density with altitude. Over a hot surface, as in the case of a desert under strong sunshine, the adjacent air may become so much rarefied that, for a certain distance upward, there is actually an increase of density with ascent, instead of the reverse. The rays of light coming to us from distant objects are bent in different directions and to various degrees by virtue of these abnormalities in the density of the atmosphere. The apparent positions of such objects depend upon the angle at which the light rays,coming from them, strike the eye of the observer. Sometimes the objects appear to be lifted far above their true positions (a phenomenon known aslooming) and sometimes depressed far below them; and occasionally local irregularities in air density produce curiously distorted images of these objects.

Most of these strange effects are known collectively asmirage. There are many varieties. There is the “desert mirage,” first made famous through the experience of Napoleon’s soldiers in Egypt. There are mirages that suspend the images of remote objects in the sky; sometimes inverted, sometimes right side up. There is the lateral mirage, occasionally seen when one looks along the face of a heated wall or cliff. Lastly, there are the complex displacements and distortions of objects known as theFata Morgana—a name originally applied to a phenomenon of this kind visible, on rare occasions, at the Straits of Messina, but now used generically for similar appearances in other parts of the world. Some of the finest examples of Fata Morgana are witnessed in the polar regions.

In the desert mirage an image of the lower part of the sky is brought down to earth and simulates the appearance of water, while the images of terrestrial objects, also depressed and inverted by the mirage, look like the reflections of the same objects upon the liquid surface. Humphreys says: “This type of mirage is very common on the west coast of Great Salt Lake. Indeed, on approaching this lake from the west one can often see the railway over which he has just passed apparently disappearing beneath a shimmering surface. It is also common over smooth-paved streets, provided one’s eyes are just above the street level.” The confusing and obscuringeffects of the desert mirage were illustrated during the fighting between the British and Turks in Mesopotamia, in April, 1917, when, according to the report of General Maude, a battle had to be suspended on account of one of these optical disturbances.

FATA MORGANA ON THE COAST OF GREENLAND(From drawing by Scoresby)

FATA MORGANA ON THE COAST OF GREENLAND

(From drawing by Scoresby)

The strong vertical contrasts in air temperature that occur in the polar regions produce many remarkable examples of mirage. The pictures and descriptions of those observed a century ago along the coast of Greenland by Captain William Scoresby, Jr., have become classical. A recent episode connected with mirage was the expedition sent north in 1913 to explore “Crocker Land,” which Peary believed he had sighted from an elevated point in Grant Land in 1906, and which for a time figured on all maps of the Arctic. The later explorers found no land at the place indicated, but they observed the same mirage that Peary had mistaken for distant hills and mountains.

Currents of air of different densities produce, through their varying effects on atmospheric refraction, the twinkling or scintillation of the stars, as well as of distant terrestrial lights. Twinkling is much more violent near the horizon than near the zenith, and more pronounced on some nights than others. The shimmering of the air over heated surfaces faces and the “boiling” of celestial objects as seen in the telescope are analogous phenomena.

INFERIOR MIRAGE(From American Museum Journal. Drawing by Chester A. Reeds.)Out-of-doors when a layer of warm rarefied air arises from contact with heated ground or warm water, occupying a position below the colder, more dense normal air, two images of a distant object may be seen—one inverted beneath the other. This is “inferior mirage” and is explanatory of the appearance of trees and their reflections, which haunts the desert traveler with the hope of water.

INFERIOR MIRAGE

(From American Museum Journal. Drawing by Chester A. Reeds.)

Out-of-doors when a layer of warm rarefied air arises from contact with heated ground or warm water, occupying a position below the colder, more dense normal air, two images of a distant object may be seen—one inverted beneath the other. This is “inferior mirage” and is explanatory of the appearance of trees and their reflections, which haunts the desert traveler with the hope of water.

SUPERIOR MIRAGE(From American Museum Journal. Drawing by Chester A. Reeds.)When a zone of warm rarefied air is sandwiched between normal air above and colder air below, a “superior mirage” of distant objects may be seen. Three images are produced, one above the other, the middle one inverted.

SUPERIOR MIRAGE

(From American Museum Journal. Drawing by Chester A. Reeds.)

When a zone of warm rarefied air is sandwiched between normal air above and colder air below, a “superior mirage” of distant objects may be seen. Three images are produced, one above the other, the middle one inverted.

In the refraction phenomena that we have thus far considered the air is the medium in which the light rays are bent and distorted. In the productionof therainbow, light undergoes refraction, dispersion (separation of the spectral colors) and reflection by passing through drops of water in the atmosphere; especially falling raindrops.

The rainbow, perhaps because it is such a common sight, is seldom observed with careful attention. Hence few people realize that there are many varieties of this beautiful meteor, and various erroneous ideas about it are prevalent. The rainbow is always seen in the part of the sky opposite the sun—or the moon, in the case of the lunar rainbow—and is high in the heavens when the luminary is low, and low when the luminary is high. Generally less than a semicircle of the bow is visible, and never more, except from an eminence. (Aeronauts occasionally see a complete circle.) The outer border of the bow is red and the inner blue or violet. Contrary to popular belief and to statements sometimes found in reference books, it is almost never possible to distinguish all seven of the spectral colors in a rainbow; four or five is the usual limit.

The ordinary orprimary rainbowhas a radius of about 42 degrees at its outer edge. Very commonly asecondaryrainbow is seen, concentric with the primary bow, and having a radius of about 50 degrees. The secondary is fainter than the primary, and its colors are in opposite order—red inside and violet outside. Additional bands of color, chiefly red and green, may often be detected adjacent to the inner edge of the primary bow and, less frequently, along the outer edge of the secondary bow. These are known assupernumerary bows. The space between the primary and secondary bows is somewhat darker than the rest of the sky.

The common rainbows differ much among themselves in the number and purity of their colors, the width of the bows, etc., these differences depending especially on the size of the raindrops. The minute drops of a fog sometimes give rise to a bow that is almost devoid of color—the “white rainbow,” or “fog bow.” The rainbows produced by the moon commonly show little color, on account of the relative faintness of the light, but the brighter lunar rainbows are often very distinctly colored.

INTERSECTING RAINBOWS(After a sketch by T. Hodge. Courtesy of Scientific American.)

INTERSECTING RAINBOWS

(After a sketch by T. Hodge. Courtesy of Scientific American.)

Reflected rainbowsare sometimes seen upon a sheet of water; and again the image of the sun, asreflected by such a surface, may give rise to both primary and secondary rainbows in the sky, which appear to intersect those produced by the sun directly. A horizontal layer of water drops below the level of the observer’s eye occasionally produces the so-calledhorizontal rainbow. This may be formed over a bedewed field or other surface (the “dew bow”); or the drops may be those of a low-lying sheet of fog, or of water deposited on a floating film of oil, or, finally, actual raindrops, seen from an elevation, such as the summit of a mountain. Horizontal rainbows formed by rain have been seen from the Eiffel Tower.

The common saying,

A rainbow in the morningIs the shepherd’s warning;A rainbow at nightIs the shepherd’s delight,

is, on the whole, well justified for the following reasons: We see the rainbow where rain is falling, while the sun is shining in the opposite part of the sky. Our rainstorms usually come from the west and pass away to the east. A morning rainbow can only be seen in the west, and indicates that rain is approaching us. An evening rainbow (ignoring lunar bows) is seen only in the east, and shows that the rain area is receding from us, giving place to clear skies.

THE CIRCUMZENITHAL ARC(From a drawing by L. Besson in La Nature.)Parts of the halos of 22° and 46°, upper tangent arc of the 22° halo, and two parhelia are also shown. The circumzenithal arc is always brightly colored.

THE CIRCUMZENITHAL ARC(From a drawing by L. Besson in La Nature.)

THE CIRCUMZENITHAL ARC

(From a drawing by L. Besson in La Nature.)

Parts of the halos of 22° and 46°, upper tangent arc of the 22° halo, and two parhelia are also shown. The circumzenithal arc is always brightly colored.

Ice crystals in the atmosphere, such as those composing the higher clouds, produce a great variety of optical phenomena, known ashalos. Some phenomena of this class are common, others exceedingly rare. Moreover, there are several theoretically possible forms of halo of which observations have never yetbeen reported, so that halo observing can be recommended to the amateur meteorologist as offering opportunities for making interesting discoveries.

Halos take the form of narrow rings of definite angular size around the sun or moon (not to be confused with the coronas, of variable dimensions, described below), rings passing through the luminary, arcs in various other positions, and roundish spots of colored or white light. They may be seen separately or in combination. In rare cases, a dozen or more different forms of halo are visible at the same time, producing a most spectacular display. One of the most remarkable displays of this kind in the history of science was seen, in different degrees of development, over the eastern United States on November 1 and 2, 1913; an event which greatly stimulated interest in the study of halos in this country. Complex halos are quite common in the polar regions; where they are seen not only in the sky, but also in the air, charged with ice particles, close to the earth.

Whenever a thin veil of cirrus or cirro-stratus clouds overspreads the sky there is a likelihood that halos will be visible. Those formed near the sun, however, frequently pass unnoticed, on account of the dazzling brightness of that luminary. Smoked or tinted glasses greatly facilitate their observation.

DIAGRAMS OF THE PRINCIPAL FORMS OF HALO(After Besson, Monthly Weather Review, July, 1914.)1. (Upper). Perspective view of the sky, showing the sun (S); ordinary halo of 22° (a); great halo of 46° (b); upper tangent arc of the halo of 22° (c); lower tangent arc of the halo of 22° (d); ordinary parhelia of 22° (e,e’); Lowitz arcs (f, f′); parhelia of 46° (g, g′); circumzenithal arc (h); infralateral tangent arcs of the halo of 46° (i); the parhelic circle (m); a paranthelion of 90° (q); light pillar, (u, u′); the observer (O). 2. (Lower). Perspective view of the sky, showing the observer (O); the parhelic circle (m): ordinary paranthelia of 120° (p); the paranthelion of 90° (q’); the oblique arcs of the anthelion (r, r′); and the anthelion (n).

DIAGRAMS OF THE PRINCIPAL FORMS OF HALO(After Besson, Monthly Weather Review, July, 1914.)

DIAGRAMS OF THE PRINCIPAL FORMS OF HALO

(After Besson, Monthly Weather Review, July, 1914.)

1. (Upper). Perspective view of the sky, showing the sun (S); ordinary halo of 22° (a); great halo of 46° (b); upper tangent arc of the halo of 22° (c); lower tangent arc of the halo of 22° (d); ordinary parhelia of 22° (e,e’); Lowitz arcs (f, f′); parhelia of 46° (g, g′); circumzenithal arc (h); infralateral tangent arcs of the halo of 46° (i); the parhelic circle (m); a paranthelion of 90° (q); light pillar, (u, u′); the observer (O). 2. (Lower). Perspective view of the sky, showing the observer (O); the parhelic circle (m): ordinary paranthelia of 120° (p); the paranthelion of 90° (q’); the oblique arcs of the anthelion (r, r′); and the anthelion (n).

The commonest halo is a circle of 22 degrees radius (the22-degree halo) about the sun or moon. When formed by the sun it generally shows a distinct reddish inner border and traces of other spectral colors. The lunar 22-degree halo usually appears colorless. This halo is visible, in whole or in part, to the attentive observer about once in three days, on an average. Less common, but by no means rare, are theparheliaor “sun dogs” of 22 degrees (calledparaselenæor “moon dogs” when formed by the moon), the beautifulcircumzenithal arc, and a few other members of the halo family. Most forms of halo are so uncommonthat their appearance is an event of some scientific importance.

The accompanying diagrams, by Dr. Louis Besson of the Observatoire de Montsouris, show the positions, with respect to the sun (or moon), of the majority of known halo phenomena. The upper diagram shows the halos that occur on the same side of the sky as the sun (or moon), and the lower those that appear on the opposite side. Most of these halos, when bright, show the spectral colors. The circumzenithal arc,h(commonly described, by the uninitiated, as a “rainbow”), and the parhelia of 22 degrees,e, e′, are especially brilliant in their coloration. Theparhelic circle, m, which sometimes extends entirely around the sky, is white, and so are a few of the rarer forms of halo.

Theupperandlower tangent arcs of the halo of 22 degrees,candd, undergo striking alterations, with changes in the altitude of the sun. When the luminary is more than about 40 degrees above the horizon, these two arcs become joined at their tips to form thecircumscribed halo, and at still greater solar altitudes this halo contracts from an elliptical to a circular form, thus blending into the 22-degree halo as shown on the next page, where the solar altitudes corresponding to the different forms of the halo are indicated. The positions of the parhelia of 22 degrees,e, e′, also depend upon solar altitude. When the sun is on the horizon these “sun dogs” are 22 degrees from the luminary, and therefore lie in the 22-degree halo; at greater solar altitudes they lie outside this halo.

The reader who wishes to acquaint himself further with the different forms of halo and the methods of observing them will find a comprehensivearticle on the subject (devoid of mathematical discussions) in the “Monthly Weather Review” (Washington, D. C.) for July, 1914.

SUCCESSIVE STAGES OF THE UPPER AND LOWER TANGENT ARCS OF THE 22° HALOWhen the sun is high they unite to form the “circumscribed halo.” (Altitude of sun shown in the center of each figure.)

SUCCESSIVE STAGES OF THE UPPER AND LOWER TANGENT ARCS OF THE 22° HALO

When the sun is high they unite to form the “circumscribed halo.” (Altitude of sun shown in the center of each figure.)

The ice crystals that produce halos consist of hexagonal plates or columns, occasionally including complications of structure, such as pyramidal bases, combinations of plates and columns, etc. These have the well-known effect of prisms in refracting and dispersing light that passes through them. It is evident that there are many possible paths for the light rays through the sides and bases of such crystals, resulting in different deflections and corresponding differences in the forms and positions of the halos produced. The attitudes assumed by the crystals as they slowly sink through the air, and theoscillations they undergo, are further points to be considered in working out the theory of each form of halo by the application of the laws of optics. Nearly all the known forms have been fully explained. A few species of halo—notably the parhelic circle (calledparaselenic circlewhen formed by the moon)—are due to simple reflection from the faces of the ice crystals, and not to refraction.

The last group of optical phenomena that we shall consider consists of those due to the process calleddiffraction, which occurs when light is bent around objects in its path, instead of passing through them, as in refraction. The process involves separation of the prismatic colors. The diffraction phenomena of the atmosphere are produced by the water drops of clouds and fog, or sometimes by fine dust.

Everybody is familiar with the nocturnal spectacle which Tennyson describes as

... the tender amber roundWhich the moon about her spreadeth,Moving thro’ a fleecy night.

This diffuse reddish or rainbow-tinted circle is called acorona. It occurs about the sun as well as the moon (though not easy to see on account of the glaring brightness of the luminary), and also about street lamps and other terrestrial lights when viewed through a misty atmosphere. Unlike the halos, it has no definite angular size. It is usually only a few degrees in radius. Small coronas are produced by large water drops and large coronas by small drops, while the largest of all coronas, known asBishop’s ring, is due to exceedingly fine dust in the atmosphere, and has been seen after great volcanic eruptions.

In its commonest form the corona consists of a brownish-red ring, which, together with the bluish-white inner field between the ring and the luminary, forms the so-calledaureole. If other colors are distinguishable, they follow the brownish red of the aureole (in the direction away from the luminary) in the order from violet to red; the reverse of the order seen in halos. Sometimes the sequence of colors is repeated three or four times.

Patches and fringes of iridescence are sometimes seen in the clouds at a greater distance from the luminary than that of the ordinary corona. Probably they are fragments of coronas of unusual size produced by exceedingly fine cloud particles.

Similar in appearance to the corona is theglory; a series of concentric colored rings seen around the shadow of the observer, or of his head only, cast upon a cloud or fog bank. Such a shadow, with or without the glory, constitutes thespecter of the Brocken, often seen from mountain tops and from aircraft. The colored circles are sometimes calledUlloa’s rings, from the name of a Spanishsavantwho observed the phenomenon among the mountains of South America in the eighteenth century and has left us a vivid description of it.

The Brocken specter, though it owes its name to legends associated with the famous German mountain where witches were once believed to assemble on Walpurgis Night, is actually less frequently witnessed there than in many other parts of the world. Whenever the sun is low on one side of a mountain and a wall of mist arises from a near-by valley on the other, the mountaineer is likely to see his shadow upon the mist. If the latter consists of fine droplets of approximately uniform size, the coloredrings will probably appear, and occasionally there is also a white fogbow outside of the glory. As all shadows cast by the sun taper rapidly (on account of the angular breadth of the solar disk), a well-defined Brocken specter can never be more than a few yards away from the observer. Its distance is, however, commonly overestimated—some observers have supposed it to be miles away!—and hence the erroneous idea prevails that the specters are of enormous size.

Rarely from a favorable point of vantage on a mountain, and very frequently from aircraft, the specter, instead of being seen on a vertical wall of mist when the sun is low, appears on a horizontal sheet of cloud below the observer when the sun is high. The aeronaut may thus observe the complete outline of his balloon or aeroplane, encircled with the rainbow tints of the glory. During the World War the appearance of the luminous rings was likened to the emblem painted on the wings of the Allied aeroplanes and was regarded by superstitious aviators as an omen favorable to their cause.

The glory is due to the light that is reflected back to the observer after penetrating the cloud or fog a little way and is diffracted by the superficial layer of drops in emerging.

The Brocken specter and the glory have occasionally been photographed.

Thoughair is but one of an unlimited number of elastic substances that transmit sound, it is the one through which sounds ordinarily reach our ears. Hence the acoustic properties of the atmosphere are of great interest to mankind.

Science deals with several kinds of “waves,” and those of the atmosphere that produce the sensation of sound are quite different from the waves of the sea. In quiet unconfined air sound travels in concentric spherical waves, consisting of successive condensations and rarefactions of the medium. Sound is not transmitted through a vacuum. A familiar laboratory experiment is to install an electric bell inside the receiver of an air pump and notice the dying away of the sound as the air is exhausted. The colossal eruptions that astronomers witness on the surface of the sun would probably be audible on earth if interplanetary space were filled with air. In the rarefied air of high mountains the intensity of sounds is much reduced. Thus we are told that on the top of Mont Blanc the report of a pistol sounds no louder than that of a firecracker at sea level.

The speed with which sound travels through air depends upon the temperature. At 32° F. (the freezing point) it is 1,087 feet per second, and at 68° F. it is 1,126 feet per second. The increase of speed with increase of air temperature is very close to 2feet per degree Centigrade, or a little more than 1 foot per degree Fahrenheit. The sounds of violent explosions travel considerably faster than ordinary sounds near the place of explosion, but slow down to the normal speed at greater distances. This is true of heavy claps of thunder. The effect is not important enough, however, to invalidate the well-known rule that, if you count seconds between the flash and the detonation and divide the result by 5, you get approximately the distance of the source of sound in miles.

Since the speed of sound varies with the temperature of the air, differences in the latter cause deviations of the paths of sound waves similar to the deviations which rays of light undergo on account of differences in the density of the air. Sound is also reflected by obstacles, in the same manner as light. Moreover, whereas light travels too swiftly to be affected by the wind, this is not true of sound. The latter travels faster with the wind than against it, and sound waves are more or less broken up by the gusts and irregularities that are a feature of most winds near the earth’s surface. For all these reasons the acoustic qualities of the air are subject to marked variations, as everybody has observed.

Unusual audibility of distant sounds is a popular prognostic of rain. The fact underlying this belief is that when the air is full of moisture it is likely to be of uniform temperature, and therefore favorable for transmitting sound.

It is impossible to assign any limit to the distance at which loud sounds may occasionally be heard. No fact of nature has yet, so far as we know, matched Emerson’s metaphor of the “shot heard round the world,” but it is literally true that the sounds oferuption of Krakatoa, in August, 1883, were heard, like the roar of distant heavy guns, in the island of Rodriguez, in the Indian Ocean, 3,000 miles from the volcano. Moreover, the atmospheric waves set up by this outburst actually made the circuit of the globe, not only once, but at least three times, and the successive journeys were registered by barometers, if not detected by human ears. During the World War gun firing in Flanders was very commonly heard at places in England 140 to 150 miles distant. Several observers also reported that pheasants appeared, from their disturbed behavior, to hear cannonading over the North Sea that was beyond the range of the human ear.

Writers have often commented on the fact that thunder cannot be heard so far as the sounds of artillery. It has been affirmed that 10 miles or thereabouts is its maximum range of audibility. As a matter of fact, however, thunder has occasionally been heard at much greater distances, up to 20 or 30 miles; but it remains true that the distance is always much less than that at which loud terrestrial sounds are audible. The reasons why this should be so are not far to seek. In the first place, the intensity of a sound depends upon the density of the air in which it is generated, and not upon that of the air in which it is heard. The air, as we know, diminishes in density upward. Balloonists thousands of feet above the earth hear with remarkable clearness sounds from the ground below, but people on the ground cannot hear similar sounds from the balloon. As thunder is mainly produced at the level of the clouds, it is subject to this peculiarity. Again, cannonading is heard at great distances only when the air is comparatively calm, and perhaps only when itis arranged in well-defined horizontal layers of such a character as to keep the sound from spreading far aloft. Very different conditions prevail during a thunderstorm; in fact the conditions are then just such as would scatter and dissipate the sound waves. Lastly, the noise of a cannon or the like comes from a single place and the energy of the disturbance is concentrated to produce a single system of sound waves; while the disturbance due to lightning is spread over the long path of the discharge.

The audibility of sounds at abnormally great distances is not usually a matter of practical importance, but the converse phenomenon—the failure of sounds to carry to normal distances—has been responsible for a great number of marine disasters on such fog-ridden coasts as those of the British Isles, eastern Canada and California. Hence some of the ablest physicists of both the Old World and the New have tried to ascertain the conditions under which this phenomenon occurs.

The scientific study of fog signals, dating especially from Tyndall’s well-known investigations at the South Foreland, in England, in 1873, and those of General Duane and Professor Joseph Henry in America, begun somewhat earlier but continued contemporaneously with Tyndall’s, has probably raised more questions than it has answered. The caprices of these signals take the shape of variations in the range of audibility—a signal may at one time carry 10 miles and at another only 2—and the formation of “zones of silence,” comparatively near the signal, within which the sound is not heard though audible at much greater distances. The silent zones are sometimes more or less permanent and are then generallydue to peculiarities of topography; but in many cases they are transient and opinions differ as to their cause or causes. Since it is only when fog prevails that fog signals are sounded (except for experimental purposes), and that vessels meet with accidents on account of the failure to hear such signals, the idea has become rooted in the public mind that these acoustic eccentricities are entirely due to fog. When, however, experiments are made in clear weather, similar phenomena are observed.

In foggy weather audibility is often better than the average, because fog prevails chiefly when the air is still and of uniform temperature, and such conditions favor the transmission of sound. Tyndall strongly denied that either fog or falling rain, snow, and hail have, as has been commonly believed, a muffling effect on sound, and he attributed the peculiar behavior of fog signals to the presence in the atmosphere of invisible “acoustic clouds,” consisting of patches of air containing irregularities of temperature and humidity. To the same cause he ascribed the occurrence of mysterious “aerial echoes,” not due to any visible object. Several recent investigators have disputed these conclusions. Thus it is asserted that when the fog signal is in fog and the observer in a clear atmosphere, orvice versa, or when the signal and the observer are in different fog banks, the fog reflects the sound very strongly. Apart from the possible effects of fog itself, the very extensive investigations made by Prof. L. V. King, of McGill University, at Father Point, Quebec, led him to conclude that the effects are chiefly due to eddies in the atmosphere. Prof. King used in his observations the latest devices forobtaining exact measurements of sound and such up-to-date meteorological apparatus as pilot balloons for measuring the wind at various levels. He discovered, among other things, that existing types of fog-signal machinery are very wasteful of energy, and he has pointed out how their “acoustic efficiency” may be much improved. Before we dismiss this subject it should be noted that submarine bells and the radio compass have made mariners much less dependent than they formerly were upon the types of signals that are affected by meteorological conditions.

“Zones of silence” on a much more extensive scale than those that disturb the operation of fog signals have been frequently observed, in recent years, in connection with great explosions, cannonading, and volcanic eruptions. The first case of this kind to attract scientific notice was that of a dynamite explosion at Förde, Westphalia, on December 14, 1903, the acoustic phenomena of which were investigated by Dr. G. von dem Borne; and among the many cases that have since been studied was that of the bombardment of Antwerp in October, 1914. Without describing these various cases separately, we may state that when reports were collected from the surrounding country to determine the places at which the sounds were audible and these reports were entered on a map, it was found that there was a large and usually very irregular area of audibility surrounding the source of sound, beyond which lay a broad, more or less circular zone of inaudibility, and finally, beginning about 100 miles from the source, there was a second large region of audibility, extending perhaps 150 miles from the source. In some cases a single sound at the source gave multiplereports (double, triple, or quadruple), chiefly in the outer zone of audibility.

In his attempt to explain these curious silent zones, Von dem Borne pointed out that the atmosphere at very high levels is supposed to consist mainly of hydrogen, in which sound travels nearly four times as fast as in the common gases of the lower air, and that sound waves ascending to such heights along a slanting course would be bent strongly toward the earth. Another student of this phenomenon, Dr. A. Wegener, who is the champion of the idea that the atmosphere contains an unknown gas lighter than hydrogen (called “geocoronium” or “zodiacon”), sees in the prevalence of this gas at high levels the cause of a similar quasi-reflection of sound waves. Probably the majority of investigators, however, believe that the effect is due chiefly or entirely to the refraction of sound by wind.

Of acoustic phenomena that belong especially to the domain of meteorology, probably thunder is the one that excites most general interest. The sudden expansion of the air along the path of a lightning discharge, due partly but probably not entirely to the heat generated, appears to be an adequate explanation of the explosive sound of thunder, though somewhat different explanations have been suggested. If the discharge is near at hand, we generally hear a single loud crash. More distant lightning is usually attended by rumbling. The common and obvious explanation of rumbling is that it is due to the arrival of the sound progressively from different points along the path of discharge, which may be a mile or more in length. A crooked path would account for reenforcements and diminutions of the sound. Another cause of irregularities in thesound is probably “interference” (combinations of waves that tend either to strengthen or to neutralize each other), especially in the case of multiple lightning discharges, such as we have described elsewhere. Lastly, thunder is further complicated by echoes from the ground and probably also from the air (not exclusively from clouds), though much uncertainty prevails concerning these aerial echoes. The sounds of thunder have been the subject of some interesting investigations on the part of an Austrian meteorologist, Dr. Wilhelm Schmidt, who has devised apparatus for making an automatic registration of the sound waves that constitute a thunderclap. He finds that there is a great preponderance of waves of very long period, including many of too low a pitch to be audible, though perceptible through the rattling of windowpanes, etc. In fact, the greater part of the energy involved is represented by these long, inaudible waves, so that one reallyhearsonly a small part of a clap of thunder.

The statement has often been made, on the authority of Humboldt, that thunder is never heard at sea, at any point far from land. This matter was investigated by the magnetic survey yachtCarnegieduring a long cruise in the Pacific in 1915. Of twenty-two displays of lightning, six were accompanied by thunder.

The late war gave prominence to certain acoustic phenomena which, though hardly mysterious, were novel to the world at large. One of those was the double report (triple in the case of an exploding shell) heard near the line of fire of large guns. This effect is due to the fact that modern projectiles travel much faster than sound. The moving projectile sets up its own waves in the air, like thoseat the bow of a steamer, which may reach the ear of the observer and produce the sensation of a sharp sound before he hears the sound coming from the mouth of the gun. Another phenomenon frequently observed when heavy firing was in progress was the appearance in the sky of rapidly moving parallel arcs of light and shade. These were generally seen against clouds, but sometimes they swept across blue sky. They probably occurred only in calm weather. These arcs were the result of the successive condensations and rarefactions of the air constituting waves of sound—visible sound waves. Their visibility was due to contrasts in the refraction of light passing through air of different densities; the same sort of refraction contrasts that cause the tremulous appearance of the air over a hot stove, for example. The same “flashing arcs” of light have been described by Prof. F. A. Perret as attending explosive volcanic outbursts at the craters of Vesuvius and Ætna.

The humming of telegraph wires has been the subject of a certain amount of discussion in meteorological circles, but without altogether satisfactory results. This sound is not, of course, caused or affected by the electric currents passing along the wire, and it is almost certainly due solely to the wind, though the suggestion has been made that it might be caused by the microseisms, or small and rapid earthquake tremors, that are so commonly registered by seismographs while imperceptible to the human senses. The humming is best heard when one’s ear is placed against a telegraph pole. Several persons have made systematic observations of these sounds from day to day, and it has often been alleged that they vary with the temperature,the movements of storms, etc., and even constitute a safe basis for weather predictions. They are sometimes heard when the air appears to be perfectly calm, but in such cases there might be some movement of the air at the level of the wires, though there was none at the lower level of the observer. From what is known about “æolian tones” (such as those of the æolian harp), it would seem that the humming requires a wind more or less at right angles to the wire, and that the pitch of the sound depends upon the force of the wind and the diameter (but not the length or tension) of the wire. For a given wire, the stronger the wind the higher the pitch of its sound.

Of all the sounds that haunt the air, probably the most mysterious are those which are best called by the generic name “brontides” (coined, in the Italian formbrontidi, by Prof. Tito Alippi from two Greek words meaning “like thunder”), though they rejoice in scores of other names in various parts of the world. Brontides take the form of muffled detonations, resembling the sound of distant cannon or peals of thunder, and are heard chiefly in warm, clear weather. The first systematic investigations of these phenomena were made in India. The fact that they were frequently reported from the neighborhood of Barisal, a town in the Ganges delta, led to their being called “Barisal guns,” under which name they were first made known to European science in 1890. A few years later they were discussed in an extensive memoir by E. van den Broeck, who had collected numerous reports of their occurrence in Belgium, especially on the seacoast, where they are known as “mistpoeffers” (i. e., “fog belchings” or “fog hiccups”). The majority of descriptions,however, have come from Italy, where the sounds appear to be extremely common, though peculiar to certain localities, and where they bear a great variety of names. In Australia the noises are called “desert sounds,” in Haiti, “gouffre,” etc. They have been reported from parts of the United States, including California and, above all, from the vicinity of Moodus, Conn., which owes its original Indian name, Morehemoodus (“place of noises”), to the brontides which appear to have formerly been much more common there than they are to-day. There is a reference in one of Lord Bacon’s works to “an extraordinary noise in the sky when there is no thunder”; apparently a description of brontides.

The source of these sounds is undoubtedly subterranean in a great many cases, though perhaps not in all. Prof. W. H. Hobbs, who has made a painstaking study of the seismic geology of Italy, concludes that the brontides of that country are due to the slow settling of the blocks of the earth’s crust; a process which, in its more abrupt and violent phases, causes definite earthquakes. Alippi believes that in order that the sounds may be heard they must be reenforced by a peculiar configuration of the ground, above or below the surface, and he attaches special importance to the effects of caverns, which he suggests act as resonance boxes in the production of audible brontides. Occasionally an apparent brontide may be due to the explosion of an unseen meteor. Lastly, a certain proportion of these thunderlike sounds, if not merely distant thunder, may be such noises as cannonading, blasting, or the like, made audible at unusual distances by the refraction of sound waves.

Someday the meteorologists of the world will join forces to produce a great encyclopædia of climate. No work of science is more sorely needed, but the magnitude that it would, ideally, assume is simply staggering.

Few people realize the multiplicity and complexity of climates. It is a common occurrence for a prospective traveler or a business man to write to a meteorological establishment requesting, for example, a description of the climate of South America. Of course, no such thing exists. A continent does not have a climate, but a multitude of climates. Even to set forth, in general terms, the more important types of climate that prevail between Cape Horn and Panama is no small undertaking. Moreover, general descriptions often fail to supply the needs of those who make inquiries about climate.

Suppose, instead of the wholesale order above mentioned, the meteorologist receives the relatively modest request to describe the climate of Buenos Aires or Rio de Janeiro. Is it easy to comply with such a request? That depends. If the information is sought by a tourist who wishes chiefly to know whether he will need light or heavy clothing at a specified season, or whether his excursions are likely to be hampered by frequent rains, we can enlightenhim in a few brief paragraphs. If the inquiry comes from a manufacturer who aspires to invade the South American markets, we must know, before replying, what kind of goods he purposes to export, and just how they are affected by climatic conditions. Are they liable to injury by high or low temperatures, dryness or humidity? Does the demand for them depend, as in the case of rubber coats, upon the prevalence of rain, or, as in the case of electric fans, upon the occurrence of hot weather during at least a part of the year? For each branch of the export trade certain elements of climate are important, and the more detailed and explicit the information that can be obtained about them the better. Suppose, again, climatic data are desired by a horticulturist who has to solve the problem of introducing a South American plant into the United States. In order to find the best environment for it in this country, he should know something about the climate of its original home. The data he requires are, however, different from those sought by the tourist or the manufacturer. Is the plant’s habitat a region where frosts occur? How long is the growing season? Is the rainfall rather evenly distributed over the year, or are there definite dry and rainy seasons? Such are some of the questions he will ask. For the purposes of medical climatology a different set of data will be sought. The astronomer, selecting a site for a new observatory, will ask about freedom from clouds, and also about the pureness and steadiness of the air that insure good “seeing.” The aviator will want information about winds and fog. And so on.

Thus it appears that climate means very different things to different people.

Climate has been variously defined as the sum total of weather, average weather, typical weather, etc., but the conception is still somewhat indefinite. We know that, while the weather of any place is subject to incessant changes, its climate persists; but we need not assume that it persists indefinitely. The geological record proves, on the contrary, that vast changes of climate have occurred in the course of long ages. In Antarctica and in Spitzbergen are found deposits of coal, constituting the débris of ancient forests such as could not exist in the climates now prevailing in those regions. There are plenty of other proofs that great climatic changes have taken place from one geological period to another; but what of changes in shorter intervals of time?

An immense amount of zeal and energy has been devoted to the study of supposed changes of climate. Evidence of such changes is sought, on the one hand, in a painstaking examination of weather records (a process often involving the tabulation of hundreds of thousands of figures), and, on the other, in the collection of geographical and historical data bearing on the question. There have been numerous reports of the gradual drying up of African and Asiatic lakes, of the discovery of ancient ruins indicating that prosperous agricultural communities once flourished in regions that are now deserts, and of various other tokens that marked vicissitudes of climate have occurred within historic times. A recent ingenious method of studying climatic variations is to measure the successive annual rings seen in cross sections of old trees. Thick rings are supposed to have been formed during periods of abundant rainfall, and thin rings when the rainfall was deficient. This method has been applied to the giantSequoiasof California, some of which are more than 3,000 years old.

The net result of a wide range of investigations appears to be that, on the whole, climate has everywhere been remarkably constant since the dawn of human history. There is much evidence that, in certain regions, there have been alternate increases and decreases—recurrent oscillations—of temperature, rainfall, etc., but there is little evidence of progressive changes in one direction.

In contrast to the uncertainty that still prevails in the scientific world on the subject of climatic changes is the confidence with which the average layman may be heard to assert that such changes have taken place within his own recollection. The popular idea that climate has changed perceptibly within a single human lifetime is a world-wide delusion, and one that has, apparently, always flourished. In the United States we hear of the “old-fashioned winter,” with its unlimited sleighing, and also of a marked increase or falling off in the rainfall in certain districts. It is an interesting fact that a century and more ago Americans were indulging in the same sort of retrospections.

In the year 1770, when Benjamin Franklin was president of the American Philosophical Society of Philadelphia, a paper was read before that society entitled: “An Attempt to account for the Change of Climate which has been Observed in the Middle Colonies of North America.” It is published in the first volume of the society’s Transactions. Barring the longs’s and the use of the word “colonies,” the greater part of it might have been addressed to the owners of automobiles and Liberty bonds. We are told of a “very observable change of climate,” remarkedby everybody who has resided long in Pennsylvania and the neighboring colonies. “Our winters,” says the author, “are not so intensely cold, nor are our summers so disagreeably warm as they have been.” These changes he ascribes to the clearing and cultivation of the country.

Another firm believer in old-fashioned winters and old-fashioned summers was Thomas Jefferson. In his “Notes on Virginia,” written in 1781, he says:

“A change in our climate is taking place very sensibly. Both heats and colds are become much more moderate, within the memory even of the middle-aged. Snows are less frequent and less deep. They do not often lie, below the mountains, more than one, two or three days, and very rarely a week. They are remembered to have been formerly frequent, deep, and of long continuance. The elderly inform me, the earth used to be covered with snow about three months in every winter.”

Samuel Williams, who published a “History of Vermont” in 1794, uses almost identical language in reference to the climate of that State. “Snows,” he says, “are neither so frequent, deep, or of so long continuance as they were formerly; and they are yet declining very fast in their number, quantity, and duration.” That these changes, he adds, “are much connected with and greatly accelerated by the cultivation of the country cannot be doubted.”

What are the facts? When the statements above quoted were written few regular records of the weather had been maintained for any length of time in this country. The earliest instrumental record was begun at Charleston, S. C., in 1730. Much information was, however, available concerning the dates of harvest, of the formation and breaking upof ice in rivers and harbors, and other events dependent upon the weather, which, if anybody had taken the trouble to collect and analyze it, would have dispelled the universal belief that marked changes of climate had recently taken place. Nowadays it is much easier to refute the common assertion that the climate has changed within the memory of living men. The meteorological history of our country for more than three-quarters of a century has been recorded from day to day by a host of careful observers in every State of the Union. The records show that, while the weather of one year has often differed strikingly from that of the next, there has been no real change in climate. “Old-fashioned” winters, for example, were neither more nor less common half a century ago than they are to-day.

Our memories of past weather mislead us, chiefly because we remember the exceptional weather and forget that which commonly prevailed. Other circumstances may contribute to the illusion. Thus many people who now live in cities, where modern appliances make them more or less independent of the weather, passed their childhood under the more primitive conditions of the country.

If climates were not fairly constant for long periods of years, it would be a waste of time to compile the climatic statistics that, as we have seen, are wanted by so many different kinds of people for so many different purposes. Such statistics are based upon past events, but their practical value depends upon the fact that, within certain limits, they are a safe guide to the future.

The climatic data for any place are a sort of digest of the meteorological observations that have been made there, special emphasis being given to thosefeatures of the meteorological record that bear important relations to the life and activities of mankind. Temperature and rainfall are the leading elements of climate; others are wind, humidity, evaporation, cloudiness, etc. We have not space to enumerate here all the kinds of data found in elaborate climatic tables; but in order to illustrate how the records of a meteorological station are utilized in compiling climatic statistics and to show what complications may arise in this process, we shall consider the question of temperature alone.

The instruments used in measuring temperature have been described in another chapter. From these instruments are obtained the current temperature of the air, the wet bulb temperature (used to compute the humidity), and the maximum and minimum temperatures of the day. Readings are made at fixed hours, known as “term hours.” At regular stations of the United States Weather Bureau the term hours for the observation of all the meteorological elements are 8 a. m. and 8 p. m., Eastern Time, and an observation of temperature, humidity, and clouds is made at noon. In most other countries tri-daily readings have been the rule, though in Europe four or more observations a day are now taken at many stations in order to supply the frequent weather bulletins required by aeronauts. Important stations are generally equipped with thermographs, which make a continuous record of temperature.

Theoretically, the mean temperature of any day is the average of 24 hourly observations, from midnight to midnight. In practice, the mean is generally computed from the observations at the term hours, or from the maximum and minimum. Having obtained the mean daily temperature for eachday of a month, the average of these values gives us the mean monthly temperature. The average of the mean temperatures for the twelve months of the year is the mean annual temperature.

These data for each day and month, and for the year—sometimes also for other intervals, such as five-day periods, or “pentads,”—are computed year after year, and eventually the values for all the years of the record are averaged to form what are called “normals.” We thus obtain, for example, for a given station, the normal temperature for January 21, the normal temperature for the month of March, the normal annual temperature, etc.

All this is a mere beginning toward the complete discussion of a body of temperature observations for the purposes of climatology. We have still to obtain from the readings of the maximum and minimum thermometers the normal maximum and minimum temperatures and range of temperature for each day, each month, and the year; also the “absolute” maxima, minima, and ranges (i. e., the extreme values that have occurred during the entire record) for corresponding intervals of time. These data furnish answers to such questions as: What was the lowest temperature ever recorded on January 21? What is the lowest on anaverageJanuary 21? What is the average range of temperature in March? What was the highest temperature ever recorded, on any day, at the station?

Having thus disposed of the extremes and ranges, we may compute what is called the “variability” of temperature, i. e., the average difference between the means of two successive days in a given month, and the corresponding average for the entire year. These data are of considerable importance in medicalclimatology. We may also compute the frequency of occurrence of various values among the temperature data above enumerated. The most frequent value is often quite different from the average value. Many climatologists compute the number of days, in an average year, on which the temperature rises to 77 degrees (Fahr.) or above (“summer-days”), and the number of days on which the temperature does not rise above the freezing point (“winter-days”). Especially valuable in agricultural regions are data of the average and extreme dates of the last frost in spring and of the first frost in autumn. These define the length of the “growing season.” Statistics of the temperature of the ground at the surface and at various depths below the surface are also of agricultural interest.

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